Transport Phenomena in Heterogeneous Catalysis

6 Transport phenomena in heterogeneous catalysis This section will not attempt to cover the more technical aspects of chemical reactor engineering. 

When external gradients correspond to substantial differences in concentration or temperature between the bulk of the fluid and the external surface of the catalyst particle, the rate of reaction at the surface is significantly different from that which would prevail if the concentration or temperature at the surface were equal to that in the bulk of the fluid. The catalytic reaction is then said to be influenced by external mass or heat transfer, respectively, and, when this influence is the dominant one, the rate corresponds to a regime of external mass or heat transfer. 

Terms such as diffusion limited or diffusion controlled are undesirable because a rate may be larger in regimes of heat or mass transfer than in the kinetic regime of operation, i.e., when gradients are negligible. 

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